1. Field of the Invention
This invention relates to a drawing die and a solid state die drawing process for orienting polymer compositions.
2. Description of Related Art
Oriented polymer compositions offer benefits in strength (that is, modulus) over non-oriented polymer compositions. Historically, polymeric films and fibers have enjoyed the benefit of increased strength through orientation in free draw processes. Free draw processes are free of physical restraints controlling drawing and offer little control over a final polymer article's shape. Therefore, free draw processes become less desirable as article cross sections become more complex.
Efforts to establish polymer orientation in articles having cross sections more complex than a fiber or thin film eventually led to solid state die drawing processes. Solid state die drawing processes require drawing a polymer composition through a solid state drawing die. The drawing die forces the polymer to converge towards a specific shape, causing alignment of polymer chains. A solid state drawing die provides more control over final article shape than free drawing processes.
Originally, die drawing processes were used to create articles having a circular cross section, articles such as rods and tubes. (See, for example, P. D. Coates and I. M Ward, Drawing of Polymers through a Conical Die, POLYMER 20, 1553 (1979)).
More recently solid state die drawing has been used to achieve large (all cross sectional dimensions greater than 1.5 millimeters (mm)) articles of oriented polymer having a rectangular cross section from a polymer billet having a square cross section. (See, for example, European Patent 1242220B1). The process necessarily directs polymer movement preferentially into the board's width dimension over its thickness dimension.
O. Richmond has developed theory around a drawing die that achieves axisymmetric deformation and streamlined flow. (See, O. Richmond, Theory of streamlined dies for drawing and extrusion. In: F. R. J. Rimrott and J. Schwaighofer, Editors, Mechanics of the solid state, University of Toronto Press, Toronto, Ontario, Canada (1968), pp. 154-167. Referred to herein as “the Richmond reference”). The objective of the Richmond reference is to develop a theory around maximizing uniformity of flow and minimizing resistance in a die while achieving axisymmetric deformation of a material. Richmond develops his theory using a cylindrical (circular cross section) article.